Many articles: are currently produced bearing a low-friction coating, among them articles used in medicine and dentistry e.g. in orthodontics. Orthodontics is a dental specialty, which aims to bring about changes in the location of abnormally positioned teeth. This is achieved by application of continuous mechanical load (orthodontic force) on the tooth, which affects the periodontal ligament (PDL) and the alveolar bone surrounding the tooth. The applied force brings about remodeling of the PDL and the bone, which enables the transposition of the tooth. The orthodontic treatment has become very common in recent decades and also adults have begun seeking this treatment for esthetic and functional reasons.
The most common orthodontic technique consists of a rigid wire (also termed “archwire”), which is inserted into slots incorporated within special attachments (orthodontic brackets) being bonded on the teeth, as illustrated in FIG. 1. The basic principle of orthodontic appliances in the archwire technique is to apply mechanical forces on teeth so that movement will occur in every desirable spatial direction. The forces are applied during the various stages of treatment by a variety of appliances, which include several kinds of archwire, ligatures, brackets and bands.
Friction among appliances used for orthodontic correction of teeth is recognized by clinicians as a hindrance to tooth movement. Friction reduces the effective force, which is applied to the tooth from the wire. In the case of sliding mechanics, excessive friction, brought about by the angle between the wire and the slot of the bracket, slows tooth movement down substantially or even halts it. A frictional type force that resists the movement of the tooth and accompanies the sliding of a tooth along an archwire is referred to as resistance to sliding (RS).
There are a number of factors that may influence the RS directly and indirectly:                1. The archwire: size, shape, material and surface.        2. The bracket: material, size and shape of the slot and its edges, surface of slot and the angle formed between the wire and the slot.        3. Ligation of the wire in the slot: elastic module, metal wire ligature and self-ligating brackets.        4. Intraoral factors: saliva, plaque and corrosion.        5. Other factors: distance between teeth and direction of the applied force.        
Over the years attempts have been made by researchers and manufacturers to reduce the friction. The problem was approached from different aspects:                1. Reduction of the size of the wire compared to the slot or the use of round wires, reduced friction to some extent but resulted in wire distortion that impaired the control of the direction of tooth movement.        2. Use of different metals (archwire or bracket) trying to reduce the coefficient of friction. The use of nickel-titanium wire has a major advantage in arch alignment due to its shape memory quality but the friction on these wires is higher compared to stainless steel (SS) wires.        3. The method of ligating the wire to the slot can reduce function. This was shown to be true with the self-ligating brackets at a 0° angle, but higher friction was recorded once the wire contacted the slot walls.        
Coating thin films of various materials onto archwires has been previously suggested as another way to reduce friction and to improve their aesthetic appearance. U.S. Pat. No. 5,288,230 describes applying a coating of diamond-like carbon (DLC) onto archwires to serve as a barrier to diffusion of nickel and chromium from the wire and also provide a hard, friction-reducing surface. U.S. Pat. No. 5,454,716 describes a coating of a plastic-ceramic composite, which is aesthetically pleasing, but is susceptible to localized abrasion over time. Another method is described in U.S. Pat. No. 6,299,438 and comprises applying a function-reducing coating containing iridium or platinum to a metal and/or ceramic dental article which is first coated with an adhesion metal layer.
Another function problem commonly encountered in dentistry is related to screw-type dental implants, between the implant and the bone walls into which the implant is inserted. Screw-type dental implants are made in two general types. The first type is a self-tapping implant that can be threaded into a pre-drilled bore in a jawbone without pre-tapping the bore. The second type is a non-self-tapping implant that requires pre-tapping of the bore. In either type, the implant has a generally cylindrical main body which bears one or more external screw threads on its outer surface. These external thread(s) engage corresponding internal thread(s) cut into the wall of the bore to provide initial stabilization of the implant in the bore.
The friction encountered by dental implants is proportional to the penetration depth of the implant into the bone, the diameter of the bore, and the hardness of the bone at the site of the bore. The torque that must be applied to insert the implant into the bore is proportional to the friction. High torque puts strains on the implant, on the tools used to place the implant in the bore, and on the bone. Furthermore, in cases where high torque is required to insert the implant, there is a greater risk of damage to the implant, the tools, and the bone. Consequently, there is a continuing need to design a screw-type dental implant which minimizes the torque needed to install it into living jawbone.